High-resolution 3D imaging is useful for improving the specificity of breast MRI since morphological characteristics, such as spiculations and heterogeneity, vary with histology. The specificity of breast MRI can be further improved by considering physiological information such as diffusion. Unlike T1-weighted contrast-enhanced imaging, it is difficult to acquire high-resolution T2- and diffusion-weighed images in reasonable scan times due to SNR and SAR limitations.
Steady-state free precession (SSFP) is a broad class of sequences in which the same RF pulse and gradient pulses are played each TR. After several repetitions, the magnetization reaches a steady state, i.e., the longitudinal and transverse magnetization are the same at the beginning of each TR. If the imaging gradients are not fully rewound, the sequence is referred to as non-balanced SSFP. For non-balanced sequences, a gradient echo signal is formed immediately following the RF pulse (the SSFP-FID signal) and another echo is formed immediately before the RF pulse (the SSFP-Echo signal). Depending on the imaging gradients used, either one or both echoes can be acquired during a single scan.
T2-weighted images are typically acquired using a spin-echo sequence with large TE and TR values to avoid T1 contamination. By acquiring images with multiple different echo times, the T2 value can be mapped on a pixel-by-pixel basis. Because fluids appear bright on T2-weighted scans, they are useful for highlighting edema and other fluid. T2-weighted imaging is commonly used in clinical scans, including evaluating cartilage health and distinguishing breast tumors from cysts.
Diffusion-weighted imaging (DWI) is sensitive to molecular motion; diffusion gradients cause moving spins to be dephased, which results in signal loss relative to stationary spins. DWI typically uses a spin-echo EPI (echo-planar imaging) acquisition. The sequence includes a slice-selective excitation followed by diffusion gradients centered around a refocusing pulse followed by an EPI acquisition. Diffusion-weighted imaging is widely used for early evaluation of stroke and has also been used in tumor imaging.
A short TE and a long TR are desirable for DWI to minimize T1 and T2 weighting in the images. However, the time required to play the diffusion gradients can lengthen TE and cause T2 or T2* weighting in the images, which is known as T2 shine-through, and can affect the quantification of diffusion. Shine-through is typically avoided by acquiring images with a minimum of three different amounts of diffusion weighting (b-values).
RARE (rapid acquisition with relaxation enhancement) is commonly used for T2-weighted imaging. 3D RARE is used clinically because it is able to acquire T2-weighted images in reasonable scan times and is relatively insensitive to inhomogeneity. RARE sequences have high specific absorption rate (SAR) because repeated refocusing pulses are used to refocus the signal in order to acquire multiple lines per excitation. T2 decay between different echoes can result in blurring since different phase encodes have different weighting. In order to achieve T2 weighting, the edges of k-space are sampled before the center of k-space, resulting in some edge enhancement. Additionally, phase errors in k-space can lead to ghosting.
DWI typically uses a single-shot EPI acquisition. The sequences are often limited to smaller matrix sizes and can suffer from severe distortions due to field inhomogeneities and ghosting due to eddy currents.
What is needed is a method to obtain high-resolution three-dimensional T2 and diffusion-weighted images using steady-state sequence in short scan times and with low SAR.